@Tony *******
the price of ALL 90 day Non-Immigrant Type visas issued inside the country is 2000baht. It is not uncommon to get a 90 day Non-Immigrant Type visa issued inside the country from a 30 day visa exempt entry or a 60 day tourist visa entry.
Most immigration offices require 15 days left on your current stamp (because the application goes under consideration for 14 days before you go back and get the Non-Immigrant Type visa inked into your passport) BUT Chiang Mai requires at least 21 days left on your current stamp.
Here's some in-country Non-Immigrant visas